Pulmonary surfactant was initially identified as a lipoprotein complex that reduces surface tension at the air-liquid interface of the alveolar compartment of the lung (Pattle, R. E. 1955. Nature 175:1125; Clements, J. A. 1957. Proc Soc Exp Biol Med 95:170). Pulmonary surfactant is synthesized and secreted by alveolar type II cells (King et al., 1973. Am J Physiol 224:788). Approximately 10% of surfactant is composed of proteins, including the hydrophilic surfactant proteins A and D (SP-A and SP-D), and the hydrophobic proteins, SP-B and SP-C (Kuroki and Voelker. 1994. J. Biol. Chem. 269:25943). SP-A and SP-D are now recognized to play important roles in innate immunity (Sano and Kuroki. 2005. Mol Immunol 42:279). SP-A and SP-D directly interact with various microorganisms and pathogen-derived components (Lawson and Reid. 2000. Immunol Rev 173:66). Moreover, by associating with cell surface pattern-recognition receptors, SP-A and SP-D regulate inflammatory cellular responses such as the release of lipopolysaccharide (LPS)-induced proinflammatory cytokines (Sano et al., 1999. J. Immunol. 163:387). LPS, derived from Gram-negative bacteria, is a potent stimulator of inflammation (O'Brien et al., 1980. J Immunol 124:20; Ulevitch and Tobias. 1995. Annu Rev Immunol 13:437). LPS molecules are engaged by the plasma LPS binding protein (LBP) (Wright et al., 1990. Science 249:1431) and transferred to CD14, a glycosylphosphatidylinisitol (GPI)-anchored protein, abundantly expressed on macrophages. LPS responses are dependent on the peripherally associated plasma membrane protein MD-2 (Nagai et al. 2002. Nat Immunol 3:667). and the membrane-spanning complex formed by toll-like receptor (TLR) 4 (Poltorak et al., 1998. Science 282:2085), through which signaling is propagated. TLRs activate four intracellular protein kinase cascades, the IB kinase (IKK)/NF-kB transcription factor cascade, the extracellular signal-regulated kinase (ERK), c-Jun NH2-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) cascades, leading to the induction of many key cytokine genes that are essential for the innate immune response (Takeda et al., 2003. Annu Rev Immunol 21:335; Medzhitov, R. 2001. Nat Rev Immunol 1:135; Barton and Medzhitov. 2003. Science 300:1524). At least one important function of SP-A and SP-D is to suppress the inflammatory response of the lung to LPS.
By weight, approximately 90% of surfactant consists of lipids. Although the lipid composition varies in different species, its major component is phosphatidylcholine (PC) (70-80%) of which nearly 80% is disaturated, consisting primarily of dipalmitoyl-phosphatidylcholine (DPPC). In addition, pulmonary surfactant contains variable amounts of phosphatidylglycerol (PG) (7-18%), phosphatidylinositol (PI) (2-4%) and phosphatidylethanolamine (PE) (2-3%) (Veldhuizen et al. 1998. Biochem Biophys Acta 1408:90). In contrast to PC, more than 50% of PG is unsaturated in many species, and in humans there is little or no disaturated PG (Schmidt et al., 2002. Am J Physiol Lung Cell Mol Physiol 283:1079; Wright et al., 2000. J Appl Physiol 89:1283). The functions of the minor phospholipid and the neutral lipid components of surfactant are largely unclear and there is a need in the art for further information regarding the roles of these components.
Previous work has provided some evidence that specific phospholipids can modulate inflammation. Oxidized phospholipid inhibits LPS-induced inflammatory responses in human umbilical-vein endothelial cells (Bochkov et al., 2002. Nature 419:77). Dioleoyl-phosphatidylglycerol (DOPG) inhibits phospholipase A2 secretion via a downregulation of NF-kB activation in guinea pig macrophages (Wu et al. 2003. Am J Respir Crit. Care Med 168:692). Treponemal membrane phosphatidylglycerol inhibits LPS-induced immune responses from macrophages by inhibiting the binding of biotinylated LPS to LBP and blocked the binding of soluble CD14 (sCD14) to LPS (Hashimoto et al., 2003. J Biol Chem 278:44205). Cardiolipin, PG and PI exhibit an inhibitory effect on LPS-induced TNF-α production by human macrophages, most likely by a blockade of the binding of LPS aggregates to LBP (Mueller et al., 2005. J Immunol 172:1091). However, very few reports have focused on the potential anti-inflammatory roles of surfactant phospholipids on either alveolar or non-alveolar macrophages. Moreover, the relationship between surfactant phospholipids and CD14 or other pattern recognition receptors has not been clearly identified.
Various studies have made connections between surfactant PG content and disease. For example, in idiopathic pulmonary fibrosis patients, some groups reported decreased unsaturated PG in surfactant (Veldhuizen et al., 1998, Biochem Biophys Acta 1408:90; Honda et al., 1988, Lung 166:293; and Saydain et al., 2002, Am J Resp Crit. Care Med 166:839). In another disease, ARDS, Schmidt et. al. have reported significant reduction in the unsaturated PG recovered in BALF (Schmidt et al., 2001, Am J Respir Crit. Care Med 163:95). The issues of cause and effect in the above diseases remain unclear.
LPS is a major cause of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) (Atabai and Matthay. 2002. Thorax 2002; Rubenfeld et al., 2005. N Engl J Med 353:1685). ALI/ARDS is a life-threatening condition in which inflammation of the lungs and accumulation of fluid in the alveoli leads to low blood oxygen levels. Over a period of 25 years the annual incidence of ALI/ARDS is 335,000, with 147,000 deaths per year. The most common risk factor for ALI was severe sepsis with a suspected pulmonary source (46%), followed by severe sepsis with a suspected nonpulmonary source (33%).
Given the severity of symptoms associated with many inflammatory conditions, including those affecting the respiratory system, there is a continued need for agents useful in controlling inflammation and thereby preventing and/or treating conditions or diseases associated with inflammation.